1. Field of the Invention
The present invention relates to a communication system and a communication method suitable for application to a radio telephone system, for example, and to a terminal apparatus used in the communication system.
2. Description of the Related Art
In a mobile communication such as a radio telephone system or the like, a multiple access in which a plurality of mobile stations (terminal apparatus or subscribers) are permitted to access a single base station is employed. In this case of a radio telephone, a number of mobile stations commonly utilize a single base station. Therefore, various communication systems have been proposed for avoiding interference between respective mobile stations. For example, a frequency division multiple access system (FDMA: Frequency Division Multiple Access), a time division multiple access system (TDMA: Time Division Multiple Access), a code division multiple access system (CDMA: Code Division Multiple Access) and so on are conventionally proposed as this kind of communication systems.
Of these systems, the CDMA system is a multiple access system in which a particular code is assigned to each of the mobile stations, a modulated wave of an identical carrier wave (carrier) is spread in spectrum with the code and then transmitted to the identical base station, and a reception side takes code synchronism based on each code to identify a desired mobile station.
Specifically, the base station occupies the whole frequency band owing to the spread spectrum, and transmits signals to respective mobile stations using an identical frequency band at the same time. Each of the mobile stations inversely spreads a signal of a fixed spread band width transmitted from the base station to extract a corresponding signal. Further, the base station distingushes each of the mobile stations by different spread codes.
In the CDMA system, communication can be achieved at every direct calling so long as a code is shared. Further, the system is excellent in secrecy of telephone conversation. Therefore, the system is suitable for a radio transmission utilizing mobile stations such as a portable telephone apparatus and so on.
In the CDMA system, it is difficult to establish a precise communication relationship between mobile stations. Therefore, each communication between respective mobile stations cannot be dealt completely separately, and hence another mobile station can become a source of interference upon communication with a mobile station. Further, data is spread within a particular frequency band in this system. Therefore, it is necessary to define a band width in advance over which the data is spread (i.e., a band width for use of transmission). Therefore, it is difficult to change the transmission band width.
The above matter will be described more concretely. FIGS. 1A and 1B shows a model in which a transmission signal of a particular user is extracted by an inverse spread from transmission signals of stations (users) which are spectrum spread with predetermined codes and multiplexed, for example. As shown in FIG. 1A, if a signal of a user U0 is to be extracted by the inverse spread from signals of eight users U0 to U7 which are multiplexed with codes, then as shown in FIG. 1B, the signal of the user U0 can actually be extracted. However, signals of other users U1 to U7 which are dealt by the same base station also become an interference source, serving as a noise. This fact results in deterioration of an S/N characteristic. For this reason, in a radio transmission employing the CDMA system, electric wave does not reach well due to the deterioration caused by the interference, which fact narrows a service area. Further, interference due to other users can be suppressed only by an amount of inverse spread gain which is obtained in a process of spectrum inverse spread. Therefore, a number of users (mobile stations) permitted to access is limited and a channel capacity becomes small.
Since a band width used for the inverse spread is usually fixed and hence the number of users from which signals can be multiplexed is limited, it was impossible to flexibly satisfy frequency allocation conditions which differ from one another depending upon countries. Therefore, only a comparatively narrow band width can be defined, which limits a maximum user rate.
Further, in the communication system to which the CDMA system is applied, there is no matching between a modulation rate of a transmission signal and a channel interval. It is necessary for a communication apparatus for the CDMA system to have both a clock generating source for modulation and demodulation and a clock generating source used for generating a channel conversion signal. This necessity complicates a circuit arrangement of the communication apparatus (such as a terminal apparatus forming the mobile station, the base station or the like), and increases a consumed power of the communication apparatus because an oscillator for generating a clock consumes a comparatively large power.
For example, in a certain radio telephone system to which the CDMA system is applied, the modulation rate of the transmission signal is 19.2 kHz and the channel interval is 30 kHz. Therefore, it is necessary to provide both an oscillator whose oscillation frequency is an integral multiple of 19.2 kHz (e.g., 19.6608 MHz which is its multiple obtained by multiplying it 1024 times) and an oscillator whose oscillation frequency is an integral multiple of 30 kHz (e.g., 15.360 MHz which is its multiple obtained by multiplying it 520 times), which requires a plurality of oscillators to be provided. Problems caused by provision of a plurality of oscillators remarkably appears to a portable mobile station arranged so as to be small-sized (a so-called portable telephone). Specifically, provision of a plurality of oscillators in one terminal apparatus is not preferable in view of attempt of downsizing the terminal apparatus, and since the terminal apparatus of this kind is driven by a battery, increase of a consumed power must be avoided in view of a battery duration.